1. Field of the Invention
The present invention relates to the exterior geometry and structural configuration for an underwater vehicle. More specifically, the invention relates to the shape of the vehicle in combination with the positioning of a plurality of sonar devices on the exterior geometry of the vehicle.
2. Description of the Related Art
Autonomous underwater vehicles (AUVs) are common scientific devices used for oceanographic research and bathymetric measurements. Because AUVs are, by definition, unmanned and autonomous, they are ideal for high-risk activities within the depths of the world's oceans. Oil and gas companies, for example, frequently use AUVs to make detailed maps of the seafloor prior to installing the infrastructures for oil rigs and pipelines. AUVs have also been used to map an area to determine whether enemy mines are present. Scientists also frequently use AUVs to study the ocean floor.
AUVs are frequently tasked to autonomously navigate in labyrinthine environments, such as rock caverns, fissures, ice pack cracks, underground flood tunnels, dam bypass tunnels, and underwater structures. In such environments, complicated terrain frequently surrounds the vehicle on all sides. As such, these conditions give rise to certain problems for traditional underwater vehicles.
First, labyrinthine environments create a true three-dimensional navigation problem for AUVs. Because of the presence of these surfaces, it is very unlikely, if not impossible, to obtain continuous or even periodic navigational updates from external navigation sources. It is common, for example, for ocean-going underwater vehicles to periodically surface for the purpose of obtaining a navigation fix from a global positioning system. It is further standard practice in the design of ocean-going underwater vehicles to have as their mission abort mechanism a system that makes the vehicle positively buoyant, so that in the event of a problem it will rise to the surface and initiate communication to indicate its status. In a complex labyrinthine environment, however, the net result would simply be the permanent loss of the very-expensive vehicle.
Traditional AUVs suffer from one additional, and frequently fatal, design flaw for working in labyrinthine environments: They are almost always torpedo shaped—that is, they almost always have long, cylindrical bodies, blunt nose cones, and a single, aft propeller. Moreover, traditional AUVS are generally designed for oceanographic research and bathymetric measurements. As such, they may employ downward-looking swath-type sonar systems (e.g., multibeam, sidescan, acoustic fresnel lens) for their science payload, and possibly a single forward-looking wide beam sonar pinger for obstacle avoidance.
The inappropriateness of this type of design for work within a labyrinthine environment can be appreciated by considering how a vehicle navigation system can be spoofed by complicated ice pressure ridges both on the overhead ice sheet as well as on the floor of an investigation zone beneath either Polar or Antarctic ice sheets—a common area of interest recently in view of the International Polar Year (IPY). AUVs have been lost in such environments and never recovered. Navigational failure in a complicated 3D environment cannot be dismissed when designing a high reliability system to work in such localities. The possible failure of torpedo-architecture AUVs in a labyrinthine environment can be attributed to many factors including lack of complete geometric knowledge of its environment (due to limited geometric sensor capabilities), lack of maneuverability (due to its long shape and aft propulsion package and therefore long turning radius), and failure of any of many single-point systems (e.g. the propeller prop becoming snagged), among many others.
For an AUV to enter into, survive, and safely return from an unexplored labyrinthine environment, it cannot rely on traditional approaches to navigation alone. Traditionally, a vehicle uses a combination of depth sensors, inertial sensors, and Doppler velocity logs (DVLs) to compute a dead-reckoned estimate of its position. With high accuracy attitude and depth sensors, most of the uncertainty in the vehicle's 3D pose (x, y, z, roll, pitch, yaw) is in the x- and y-directions. Because dead-reckoning error will compound with time, and there will be no opportunity for pose correction from an external source—GPS, for example, will not penetrate ice nor rock—the vehicle must use something else to obtain a positive lock on its pose. In fact, it is crucial for minimization of uncertainty that the geometric sensors provide the most accurate possible true registered values of the local geometry within the 4π steradian solid viewing angle around the vehicle. Stated in simpler words, the vehicle's sensors must look simultaneously in all directions about the vehicle, preferably with data coming from sensor pointing directions radially separated by approximately uniform solid angles.
There are essentially six variations of sonar transducers: (1) single beam, unfocused; (2) Mills Cross multibeam; (3) interferometric sidescan; (4) acoustic fresnel lens; (5) 3D multibeam; and (6) narrow beam, focused. The first sensor (single beam, unfocused) is essentially the old spherical wave depth sounder and it will produce fictitious results from all but a flat, planar surface. The next three (multibeam, sidescan, acoustic fresnel lens) are swath imaging systems that scan a thin, wide fan from the vehicle and the vehicle must translate to produce a valid image. While indispensible for bathymetric surveying of essentially 2½ D ocean floor topography, they are bulky instruments that cannot effectively (both physically and from a cost perspective) cover a 4π steradian solid viewing angle about a vehicle. Further, in a labyrinthine environment, these approaches are particularly susceptible to multipath spoofing and will therefore generate false input data leading to loss of pose lock.
3D multibeam imagers can simultaneously image a large number of points within a nominal field-of-view (FOV) of around a 50×50 degree solid angle, and, therefore, theoretically could be discretely arrayed around a vehicle to obtain the required 4π steradian real-time geometry. These devices, however, rely on a reasonable degree of phase coherence to operate and multipath returns from a labyrinthine environment serve to corrupt phase coherence.
The last sensor type—focused and stabilized narrow beam sonar—provides a characteristic that is uniquely-suited to obtaining true geometry in a labyrinthine environment. The very narrow (less than two-degree solid angle) beam ensonifies a very small area relative to the reference sphere of the vehicle and the returns are easily distinguished from other transducers, even in a highly-irregular labyrinthine environment, provided the transducer solid angle separation is greater than about ten degrees between sensors. Further, and equally importantly, this class of sensors (single transducer, narrow beam) can be split from the normal method of sonar manufacture (in which the transducer contains its own case with associated electronics as an integrated, blocky housing) and divided into component elements in which the transducer (projector/hydrophone combination) can be produced as a low profile flat disk that can be connected to a remote digital signal processor (DSP) by a long conductor (up to 2.5 m or more depending on frequency of operation). This very significant factor allows one to “pave” the exterior hydroshell surface of the invention with a uniform solid angle field of sonar rangers.
Finally, because each transducer can be provided with its own DSP chip within a rugged remote electronics housing protected in the core of the vehicle, and because the signals coming back only represent the returns from a very small ensonified area, the ability to reject, in real-time, any multipath or spurious echoes is very powerful. As a result, fifty to one-hundred good, true real-time data returns spread evenly over a 4π steradian solid viewing angle relative to the vehicle centroid leads to reliable convergence far better than tens of thousands of points that are corrupted by multipath.